When I voted I didn't know the year. I should have guessed from the choices. Only the 78-80 300SD had the 722.1 trans, which did have a reverse band (all newer models don't).
You probably have an adjustment problem. The servo piston also breaks and/or the seal blows. All of the above can be done with the trans in the car.
Bosch Master, ASE Master, L1
33 years MB technician